How do I install Blenrig Addon in blender 2.9
From BlenRig 5 Documention

Download the BlenRig .zip file.
Blender allows to install .zip addons easily. Open User Preferences,
go to the Add-on tab, and press &quot;Install From File&quot;. Then browse to
the download directory where the zip is.

After this is done, the add-on should appear in preferences available for use.


A new rig is added via Shift+A&gt;Armature&gt;Blenrig 5 Biped Rig
Also, from the docs:

You will find the rig in the Object Add Panel, under the Armature
Menu. In the View3d Sidebar you will find all the animation controls.
In the Armature Data Panel you will find all the rigging related
tools.